1. Field of the Invention
The invention relates to an improvement in a method and device for the diagnosis and treatment of speech disorders and more particularly to the dynamic measurement of the functioning of the velum in the control of nasality during speech.
2. Description of the Related Technology
A. Velar Control and Oronasal Valving in Speech.
During speech or singing, it is necessary to open and close the passageway connecting the oral pharynx with the nasal pharynx, depending on the specific speech sounds to be produced. This is accomplished by lowering and raising, respectively, the soft palate, or velum. Raising the velum puts it in contact with the posterior pharyngeal wall, to close the opening to the posterior nasal airflow passageway.
This oronasal (or velopharyngeal, as it is usually referred to in medical literature) passageway must be opened when producing nasal consonants, such as /m/ or /n/ in English, and is generally closed when producing consonants that require a pressure buildup in the oral cavity, such as /p/, /b/ or /s/. During vowels, and also the vowel-like sonorant consonants (such as /l/ or /r/ in English), the oronasal passageway must be closed or almost closed for a clear sound to be produced, though in some languages an appreciable oronasal opening during a vowel can have phonemic significance and thus be required for proper pronunciation. The first vowels in the words “francais” and “manger” in French are examples of such nasalized vowels. In addition, vowels adjoining a nasal consonant are most often produced with some degree of nasality during at least part of the vowel, especially if the vowel is between two nasal consonants (such as the vowel in “man” in English).
There are many disorders that result in inappropriate oronasal valving, usually in the form of a failure to sufficiently close the oronasal passageway during non-nasal consonants or non-nasalized vowels. Such disorders include cleft palate and repairs of a cleft palate, hearing loss sufficient to make the nasality of a vowel not perceptible to the speaker, and many neurological and developmental disorders. The effect on speech production of insufficient oronasal closure is usually separated into two effects, namely, the nasal escape of pressurized oral air, termed ‘nasal emission’, that limits oral pressure buildup in those speech sounds requiring an appreciable oral pressure buildup (as /p/, /b/, /s/ or /z/), and, secondly, the incomplete velar closure during vowels and sonorant consonants that is often referred to as ‘nasalization’. (See R. J. Baken and R. F. Orlikoff, Clinical Measurement of Speech and Voice, second edition, 453 et seq. (Singular, Thomson Learning, 2000)). The terminology used here is that suggested by Baken and Orlikoff, supra, who also prefer to reserve the term ‘nasality’ for the resulting perceived quality of the voice.
Since the action of the velum is not easily observed and the acoustic effects of improper velar action are sometimes difficult to monitor auditorally, there is a need in the field of speech pathology for convenient and reliable systems to monitor velar action during speech, both to give the clinician a measure of such action and to provide a means of feedback for the person trying to improve velar control.
B. Previous Methods for Measuring Velar Function
The various methods for monitoring velar function according to the present art can generally be also divided into two categories, according to the aspect of nasality being measured: (a) those methods that measure velar control during consonants requiring an oral pressure buildup, and (b) those methods that measure velar control during vowels and sonorants. In this application, for brevity we hereafter use the term ‘vowel’ to refer to both vowels and sonorants (vowel-like consonants).
The field of the invention relates to a commonly used method for measuring the nasalization of vowels by recording the sound energies (either radiated acoustic pressure or radiated acoustic volume velocity, or airflow) separately emitted from the nose and mouth, usually in conjunction with the placing of a sound barrier held against the upper lip to improve the separation of the nasal and oral sounds, with microphones placed above and below the barrier, respectively. In U.S. Pat. Nos. 3,752,929 and 6,974,424, the nasal and oral energies are recorded in the form of the respective radiated acoustic pressures, while in U.S. Pat. No. 6,850,882, the nasal and oral sounds are recorded in the form of the respective volume velocities, using a two-chamber pneumotachograph mask having a separating membrane contacting the upper lip. The respective nasal and oral signals are suitably filtered and a ratio taken of the nasal to oral energies. This ratio is commonly referred to as vowel ‘nasalance’, and can be presented as either the Nasalance Ratio (nasal energy divided by oral energy) or Percent Nasalance (nasal energy divided by the sum of nasal and oral energies). The term ‘nasalance’ can be used to refer to either of these measures, or to any third measure mathematically derived by comparing oral and nasal acoustic energies.
Though nasalance is valuable as an objective measure of the degree of a lack of velar closure, in all methods for measuring vowel nasalance, there is a marked dependence of the value obtained on the vowel being spoken, even with the same degree of closure for each vowel. (Lewis K E, Watterson T and Quint T, “The effect of vowels on nasalance scores”, Cleft Palate-Craniofacial Journal, 37: 584-589 (2000); Gildersleeve-Neumann, and Dalston, “Nasalance scores in noncleft individuals: why not zero?” Cleft Palate-Craniofacial Journal, 38: 106-111 (2001)) This variation is presumed to be caused by the fact that for vowels having a constriction in the vocal tract anterior to the velum, there is a higher acoustic energy in the oral pharynx and thus a higher energy emitted nasally for the same degree of velar opening. Thus for a given velar opening, the vowel /i/ as in “bead” has a higher value of nasalance than the vowel /a/ as in “bob”. For example, Baken and Orlikoff, supra, in their summary of the literature, report that the nasalance recorded for normal-speaking children according to the present art can vary from approximately 7% in a non-nasalized /a/ vowel to 17% in a non-nasalized /i/ vowel (with both vowels measured in a /p/ phonetic context that minimizes nasalization of the vowel). This range is consistent with the measurement in FIG. 5 below for an adult subject. This variation of 10% according to the vowel being spoken occurs even if there is no velar opening, since with no velopharyngeal opening there is still a small amount of nasally emitted energy that is caused by vibrations of the velar tissue, and this energy is apparently greater for the /i/ vowel.
Errors of 7% or 17% are quite significant in that the total range for the nasalance of vowels is much less than 100%. This range is theoretically from zero to only about 40%. Consequently, the nasalization of a specific vowel can be expected to raise its nasalance score by no more than about 30%, depending on the degree of nasalization (velar opening). The limitation on the total range for vowels can be better understood by considering that if the velum is fully lowered during a typical vowel, resulting in an unnaturally large degree of nasalization, the Percent Nasalance should be close to 50%, say between 40% and 60%, since there is roughly equal energy emitted from the oral and nasal passageways. (This assumes no abnormal constriction of the nasal passages, as may be evidenced with the swollen mucous membranes accompanying nasal congestion.) Values much above 50% would be expected only during nasal consonants, when the oral passageway is occluded. Thus, when a device for measuring nasalance is constructed according to the present art, the nasalance for a totally non-nasalized /i/vowel could be similar to that recorded for a moderately nasalized /a/ vowel.
In addition to the variation with the vowel spoken, nasalance values obtained using devices constructed according to the current art are affected by acoustic energy from one channel crossing over into the other channel because of an incomplete acoustic separation of the channels. Thus the lowest values of nasalance obtained tend to be about 5% to 7%, instead of near zero, as otherwise expected, and the values of nasalance recorded in properly articulated nasal consonants tend to vary from approximately 90% to 95%, instead of being closer to the theoretically expected 100%.
The variation of vowel nasalance according to the vowel spoken can be reduced somewhat by suitably filtering the oral and nasal signals or by using airflows instead of pressures as the variables to be measured, and the effect of acoustic crossover can be decreased by improving the acoustic separation means, however, none of these methods have shown the ability to eliminate or make negligible these distortion effects. In their comprehensive review of attempts to use nasalance as a measure of velar closure and nasality, Baken and Orlikoff, supra, p. 466, conclude that “It also remains unclear how nasalance is affected by the physical characteristics of the oral and nasal cavities . . . and by the phonetic demands of the spoken utterance.” These authors review a number of attempts to devise testing procedures that circumvent, or at least take into account, the variability of the nasalance measure, by limiting testing to a fixed phonetic sequence, as a particular sentence, passage or nonsense syllable sequence.